The present invention relates generally to electrical microcircuit structures with silicon nitride passivation, and more particularly to improved structures that allow included thin-film components to be laser trimmed without damaging the passivation coating.
In the manufacture of thin-film and monolithic hybrid microcircuits, passive circuit elements such as resistors and capacitors are prepared from films of materials only a few thousand angstroms thick. The films usually are deposited by vacuum evaporation or sputtering, with the necessary patterning being accomplished before, during, or after deposition. As a final step before packaging, a protective overcoating or passivation film may be applied to the circuit. A good passivation coating is especially necessary if the microcircuit will not be sealed in a hermetic enclosure. Silicon nitride (Si.sub.3 N.sub.4) is used extensively as a passivation material because of its high resistivity and dielectric strength, excellent chemical resistance, and superior electrical and thermal stability.
Even with well-controlled processes, the values of initially fabricated thin-film components typically fall within a 5-15% tolerence range. More accurate values are achieved by physically removing portions of the components in a subsequent trimming operation. Airborne abrasive, electric arc, and laser beam trimming systems have been developed for this purpose. Laser trimming systems have a number of significant advantages, including greater speed, accuracy, and cleanliness. In addition, they can be used under computer control to adjust circuit components while their values are being measured.
Components may be laser trimmed after the passivation film is applied if a laser operating in the visible or near infrared region is used. By so doing, a completed circuit can be adjusted for optimum operation during active, functional testing.
In the past it has not been possible to trim certain thin-film components in silicon nitride-passivated circuits without damaging the nitride layer. For example, during trimming of Nichrome and other nickel- or chromium-containing films, voids and cracks in the passivation layer are produced, forming an entry point for contaminants.
Because of the superior protection provided by silicon nitride, there is a need to provide a way to laser trim thin-film components containing nickel, chromium, or other metals in hybrid circuits that incude a Si.sub.3 N.sub.4 passivation layer.